.arcconfig

tests: adjust merge tool config in test-largefiles-update.t

Previously, the `ui.merge=internal:fail` configuration was shadowed by the the
`HGMERGE=internal:merge` environment variable. We would like to remove this
usage of `HGMERGE` variable because it is error-prone (eg: this very test). We
start by cleaning up test independently to avoid large churn when the change
happens.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}